Negative feedback amplifiers



June 26, 1956 Q wHlTE ET AL 2,752,433

NEGATIVE FEEDBACK AMPLIFIERS Filed Sept. 21, 1950 Laventopa EL, C.lsfhz'il'e FTC G'bILLLUGLL United NEGATIVE FEEDBACK AMPLIFIERS Claimspriority, application Great Britain September 26, 1949 3 Claims. (Cl.179-171) This invention relates to negative feedback amplifiers.

In negative feedback amplifiers which are required to amplify a widerange of frequencies, such for example as amplifiers utilised inscanning circuits in television equipment, difficulty is sometimesexperienced because the amplifier may include elements, such astransformers, which cause considerable phase shift of higher frequencycomponents relative to lower frequency components in said range offrequencies. This may result in the feedback becoming positive for thehigher frequency components thereby rendering the amplifier liable toinstability at the frequencies on which the feedback becomes positive.

In the copending United States application Ser. No. 100,694, filed June22, 1949, by Eric Lawrence Casling White, and which has issued as PatentNo. 2,652,459 on September 15, 1953, there is described an improvedamplifier construction with a view to reducing the difficulty referredto in the preceding paragraph and there is also described a thermionicvalve having at least a cathode, a control electrode, and an outputelectrode, an input circuit for applying signals to be amplified to saidcontrol electrode, a transformer having a primary winding in circuitwith said output electrode, an output circuit including a secondarywinding of said transformer and a resistance in series with saidsecondary winding, filter means for applying signals set up across saidresistance to aid input circuit to afford negative feedback to saidinput circuit, a resistance in the cathode lead of said valve, andfilter means for applying signals set up across said second resistanceto said input circuit to afford negative feedback to said input circuit,said first and second filter means having complementary frequencycharacteristics with the pass-band of the first means in a frequencyrange in which feedback signals from said output circuit haveappropriate phase to afford negative feedback to said input circuit, andwith the pass-band of the second means in a frequency range in whichsignals from said output circuit would afiord positive feedback if fedto said input circuit, and said first and second resistances beingdimensioned to set up feedback signals of substantially equal amplitudesat least at the changeover frequencies of said filter means.

In the particular form of the invention illustrated in the aforesaidapplication the amplifier is employed in the frame scanning circuit oftelevision equipment and comprises a valve 1 to the control electrode ofwhich the scanning waveform is applied after amplification in a pair ofcathode coupled valves 5 and 6. A transformer 2 in the anode circuit ofthe valve 1 feeds the scanning waveform to the scanning coils 4 in thesecondary circuit of the transformer. A first feedback path is providedfrom the resistance 16 in the secondary circuit of a transformer 2, andit comprises a resistance 17 and condenser 18 and a second feedback pathis provided from the cathode of the valve 1 via condenser 19, the

feedback signals for this path being set up across a resistance 20 inthe cathode lead of the valve 1. The feedback signals from both pathsare fed to the control electrode of the valve 6 in the appropriatephase.

The object of the present invention is to provide a modified formofamplifier which displays characteristics similar to the amplifierdescribed in the aforesaid application and which displays advantages incertain applications of such amplifiers.

According to the invention there is provided a negative feedbackthermionic valve amplifier comprising an input circuit, an amplifyingpath leading from said input circuit and including a thermionic valvehaving an output electrode, a complex load impedance connected to saidoutput electrode, filter means leading to said input circuit, means forapplying voltage variations set up across said complex impedance to saidfilter means, said filter means being dimensioned to transmit voltagevariations in a relatively low frequency range and to attenuate voltagevariations in a higher frequency range, a resistance connected in aspace current path of said valve, second filter means leading to saidinput circuit, and means for applying voltage variations set up acrosssaid resistance to said second filter means, said second filter meansbeing dimensioned to transmit voltage variations in said higherfrequency range and to attenuate voltage variations in said lowerfrequency range, and the respective filter means being connected toproduce negative feedback to said input circuit in response to thetransmitted voltage variations.

In order that the said invention may be clearly understood and readilycarried into effect, the same will now be more fully described withreference to the accompanying drawings in which:

Figure 1 illustrates one example of the present invention.

Figure 2 illustrates a modification of Figure 1.

Figure 3 illustrates another modification of Figure 1.

Referring to the drawing the amplifier illustrated in Figure l isembodied in a frame scanning circuit of television equipment and is ofthe same general construction as the amplifier illustrated in theaforesaid application, corresponding parts being indicated by the samereference numerals. It comprises a pair of cathodecoupled amplifyingvalves 5 and 6, the input signals being applied to the control electrodeof the valve 5 and the valve 6 having a suitable anode load, the signalsset up across which are fed to the control electrode of an outputvalve 1. This latter valve feeds the scanning coils 4 via an outputtransformer denoted in general by the reference character 2, the primarywinding of the transformer being connected in the anode lead of thevalve. The input signals consist of sawtooth waveform potentials offrame frequency and are applied to the control electrode of the valve 5via a large resistance 23. Furthermore the transformer 2 has twosecondary windings 24 and 25, the scanning coils 4 being fed from thewinding 24. The winding 25 is earthed at one end and coupled forfeedback, via a large resistance 26, to the control electrode of thevalve 5. A second feedback path is provided from a small feedbackresistance 27 in the anode lead of the valve 1 to the control electrodeof the valve 5, the anode end of the resistance 27 being coupled via acondenser 28 to a tapping 29 on the resistance 26 located above theaforesaid virtual earth point. The other end of the resistance isdecoupled to earth via a condenser 30.

It will be appreciated that the load impedance presented to the anode ofthe output valve 1 is complex since the coils 4 and transformer 2 have aresistive component and a reactive component of impedance. Moreover, the

reactive component of the impedance will change with frequency, givingrise to phase displacements in the voltage variations set up across theimpedance. In operation of the arrangement, the lower frequencycomponents of the voltage variations set up across the secondary winding25 have effectively the same phase as the current variations in theprimary circuit of the transformer and hence the voltage variations setup across the feedback resistance 27, since at the lower frequenciesencountered in a frame scanning circuit, the impedance of thetransformer 2 in practical cases is mainly resistive. For lowerfrequency components feedback is received at the control electrode ofthe valve 5 mainly from the winding 25 via the resistance 26, the phaseof such feedback being appropriate to provide negative feedback. Atlower frequencies the impedance of the condenser 28 is so high aseffectively to prevent feedback being received from the resistance 27.The higher frequency components of the voltage variations set up acrossthe winding 25 are displaced in phase relatively to the correspondingcurrent variations in the primary circuit of the transformer, and hencein relation to the higher frequency components set up across theresistance 27, and are not suitable for negative current feedback.However, at such frequencies the impedance of the condenser 28 is lowand a low impedance path is provided from the tapping 29 to ground viathe condenser 28, resistance 2'7 and decoupling condenser 30. Thereforethe higher frequency components of voltage variations set up across thewinding 25 are substantially attenuated at the tapping 29 and renderedineffective to cause feedback to the control electrode of the value 5and the liability to instability due to the phase shift produced in suchcomponents by the transformer 2 is substantially reduced. On the otherhand, due to the low impedance of the condenser 28 at such frequenciesthe higher frequency components of the voltage variations set up acrossthe resistance 27 are less attenuated and afford feedback to the controlelectrode of the valve 5 and since the transformer 2 is not efiective toproduce a phase shift of these components, their phase is appropriatefor providing negative feedback. Moreover, the tapping 29 is so locatedthat the voltage variations received at the tapping 29 from theresistance 27 at the higher frequencies have the same amplitude as thecorresponding voltage variations received at the tapping 29 from thewinding 25 at lower frequencies, so that negative feedback, as if from asingle feed point, is applied over a wide range of frequencies and theamplifier is maintained stable throughout a wider range of frequenciesthan would otherwise be the case. In effect, the feedback to the inputcircuit of the amplifier is derived from the winding 25 and theresistance 27 via filters having complementary frequencycharacteristics, that is to say the transmission characteristic of oneof the feedback paths is the same as the attenuation characteristic ofthe other feedback path, this result being inherent in the directconnection of the high potential ends of the winding 25 and of theresistance 27 by means of the condenser 28 and the upper part of theresistance 26 in series, as shown in the drawing. Moreover, thepass-bands of the separate feedback paths are such that the feedback isderived from the winding 25 in the frequency range in which the voltagevariations set up across the winding are not subjected to such phaseshift with respect to currents in the primary winding as would give riseto oscillations, whilst for higher frequency components the feedback isderived from the resistance 27, the current in which is not subjected tosuch phase shift. In operation of the arrangement, a virtual earth pointoccurs near the lower end of the resistance 26, the junction point ofthe resistances 23 and 26 being a point of small potential excursion andthe valve operating with a low input impedance.

Instead of providing the resistance 27 in the anode lead of the valve 1,in series with the primary winding of the transformer 2, it may beprovided in the lead to the screen electrode of the valve 1, in eithercase the resistance 27 being connected in a space current path of thevalve 1, that is a path traversed by at least some fraction of thecathode current of the valve 1. In this case since the currenttraversing the resistance 27 will only be a fraction of that traversingthe primary winding of the transformer, the magnitude of the resistance27 will require to be increased. This modification is illustrated inFig. 2. Moreover, in some cases the winding 25 of the transformer 2 maybe dispensed with, and the resistance 26 connected directly to the lowerend (in the drawing) of the primary winding of the transformer 2. Thismodification is illustrated in Figure 3.

What we claim is:

l. A negative feedback thermionic valve amplifier comprising an inputcircuit, an amplifying path leading from said input circuit andincluding a thermionic valve having an output electrode, a complex loadimpedance connected to said output electrode, filter means leading tosaid input circuit, means for applying voltage variations set up acrosssaid complex impedance to said filter means, said filter means beingdimensioned to transmit voltage variations in a relatively low frequencyrange and to attenuate voltage variations in a higher frequency range, aresistance connected in a space current path of said valve, secondfilter means leading to said input circuit, and means for applyingvoltage variations set up across said resistance to said second filtermeans, said second filter means being dimensioned to transmit voltagevariations in said higher frequency range and to attenuate voltagevariations in said lower frequency range, and the respective filtermeans being connected to produce negative feedback to said input circuitin response to the transmitted voltage variations.

2. A negative feedback thermionic valve amplifier comprising an inputcircuit, an amplifying path leading from said input circuit andincluding a thermionic valve having an output electrode, an outputcircuit including a transformer having a primary winding connected tosaid output electrode, a feedback resistance connected in a spacecurrent path of said valve, a filter resistance connected from a windingof said transformer to said input circuit, and a filter condenserconnected from said feedback resistance to an intermediate point on saidfilter resistance, said filter resistance and said filter condenserbeing dimensioned to transmit voltage variations from said winding in arelatively low frequency range, to attenuate voltage variations fromsaid winding in a higher frequency range and to produce complementarytransmission and attenuation of voltage variations from said feedbackresistance, and said filter resistance and filter condenser beingconnected to produce negative feedback to said input circuit in responseto the transmitted voltage variations.

3. A negative feedback amplifier comprising an amplifying path includinga thermionic valve having an output electrode, a series feed resistanceleading to said path for feeding signals to be amplified to said path, acompleX load impedance fed from said output electrode, a feedbackresistance of low impedance compared with said complex impedanceconnected in a space current path of said valve, a filter resistanceconnected from said complex impedance to the end of said feed resistancenearer said path, and a filter condenser connected from said feedbackresistance to an intermediate point in said filter resistance, saidfilter resistance and said filter condenser being dimensioned totransmit voltage variations from said complex impedance in a relativelylow frequency range, to attenuate voltage variations from said compleximpedance in higher frequency range and to produce complementarytransmission and attenuation of the voltage variations from saidfeedback resistance, and said filter resistance and said filtercondenser being connected to produce negative feedback to said inputcircuit in response to the transmitted voltage variations.

References Cited in the file of this patent UNITED STATES PATENTSWorcester June 17, 1941 10 6 Levy Dec. 24, 1946 Beurtheret July 28, 1953FOREIGN PATENTS France May 16, 1938 France Dec. 7, 1938 France Apr. 4,1945 France Sept. 4, 1945 Great Britain Nov. 19, 1941 Great BritainSept. 14. 1943

